Arvind Kejriwal (C), leader of the newly formed Aam Aadmi (Common Man) Party, shakes hands with a party volunteer outside the party office in New Delhi October 1, 2013.

On a balmy autumn evening at a neighborhood market in Delhi, a slight man in rolled-up shirtsleeves and loose pants exhorts a crowd of several hundred people to do something they have never done before: vote for a brand new party in state elections scheduled for December.

“The revolution will start in Delhi and spread throughout the country,” says 45-year-old anticorruption activist Arvind Kejriwal in Hindi. His voice, amplified to a deafening pitch, drowns out traffic on the busy road nearby. The crowd listens, rapt.

As Mr. Kejriwal speaks, two volunteers of his Aam Aadmi (Common Man) Party wend their way through the crowd with a bed sheet, gathering donations in mostly small bills. The message: AAP will rely on ordinary voters, not fat-cat donors, to fund its campaign.

AAP’s quixotic bid for power could alter the face of Indian politics. Should the party perform as well as expected in the Delhi election—most polls show it in third place behind the ruling Congress Party and the opposition Bharatiya Janata Party—the reverberations will be felt across the country. It will show for the first time that a party rooted in a middle-class ethos, with a message based on ideas rather than the identity politics of caste or religion, can compete in the hurly burly of Indian politics.

Yet AAP doesn’t represent unalloyed progress. Mr. Kejriwal’s speeches show how the default setting for Indian parties remains leftist populism. That even urban voters in one of the country’s richest pockets must be wooed with promises of freebies and handouts—leavened with a large dollop of resentment against business—illustrates the failure of India’s politicians and intellectuals to build a broad constituency for market-friendly economic policies.Enlarge Image

AAP represents the middle-class anger against public corruption that erupted in street protests two years ago. India’s series of multibillion-dollar corruption scandals spanning coal, telecoms and real estate created the conditions for the new party’s birth.

Mr. Kejriwal gives a speech promising to enact a tough new anticorruption law in the very fairgrounds where two years ago his former colleague, 76-year-old activist Anna Hazare, electrified India’s imagination with a 13-day fast. Mr. Hazare has since split with Mr. Kejriwal over his decision to embrace party politics, but the AAP leader rarely misses an opportunity to remind voters of his former association with the older man.

The fledgling party has already set itself apart from the competition through its savvy use of both mainstream and social media. Its marketing campaign seeks to use innovation to compensate for a small war chest, and it is relying on idealistic volunteers to fan out to every home in Delhi. The party has draped banners over flyovers and encouraged homeowners to plaster their walls with posters of themselves and Mr. Kejriwal.

The odds in Indian elections rarely favor outsiders, who are invariably outspent and out-organized by bigger parties. But even a strong third-place finish by AAP in Delhi—say, 10 seats out of 70—could signal that politics as usual is over in urban India. It will declare that mainstream parties can no longer afford to ignore middle-class sentiment.

Taken together with a recent Supreme Court decision to ban politicians convicted of crimes from holding public office or contesting elections, the AAP’s rise signals a growing clamor for a cleaner public life. As the country grows richer and more educated, its citizens are beginning to demand more of their elected representatives.

Yet beneath the surface, both AAP and Mr. Kejriwal aren’t so appealing. On the stump, Mr. Kejriwal promises to slash electricity rates in half while guaranteeing round-the-clock power. (He claims private power companies and politicians are in cahoots to keep prices artificially high.) He also promises free water to the poor and thousands of new government schools that will be “better than private schools.” He doesn’t say how he will pay for any of this.

So beneath the patina of a new crusading zeal lies a populist message similar to those of so many professional politicians Mr. Kejriwal routinely derides.

What would be the best outcome, then, in December’s elections? Not a crushing defeat for AAP, which would bode poorly for those who are fed up with corruption, and want politics to transcend caste and religion. India needs its best-educated citizens to engage more deeply with the democratic process, instead of seceding from it by refusing to vote or by holding the entire political class in contempt.

Yet an AAP victory would also be a disaster, putting one of India’s richest and most dynamic states in the hands of a rabble rouser who believes that the country’s 1991 reforms—which have lifted tens of millions out of poverty—benefited only a handful of well-connected businesses.

Instead, Delhi’s middle-class voters should choose the middle path. They ought to give Mr. Kejriwal enough support to jolt Congress and the BJP into taking their concerns more seriously. But they shouldn’t hand Mr. Kejriwal power until he develops a message that embraces growth and prosperity, not just prison terms for the corrupt and handouts for most others. Delhi deserves better than an angry man with a big stick.Mr. Dhume is a resident fellow at the American Enterprise Institute and a columnist for WSJ.com. Follow him on Twitter @dhume.